Modified and unmodified starch products are extensively used for a variety of non-food and industrial applications. They have, traditionally, been used to size or finish textiles and papers, as adhesives (e.g., corrugated and laminated paper boards, remoistening gums, wallpapers, etc.), flocculants, binders (e.g., foundry core binders), fabric printing aides, thickeners and many other divergent non-food and industrial applications.
In the coating and shaped article manufacture, the trade presently relies upon synthetic polymeric materials which are primarily produced from petrochemical raw materials. Petrochemicals are a depletable natural resource. Within recent years, world-wide demand for petroleum-based products has adversely affected the cost and availability of synthetic polymers. Starches are readily available and replenishable with each crop year. Potential starch product usage would substantially increase if it were possible to alter or correct certain inherent defects which have heretofore rendered starch products unsuitable for coating and/or shaped article applications.
Starches are inherently unstable against physical, chemical, bacterial and enzymatic degradation. Starches vary in amylopectin and amylose content. Waxy starches consist essentially of amylopectin with only trace amounts of amylose. Corn starch and other conventional starches such as tapioca, potato and wheat typically contain 16-24% amylose (dry solids weight basis) with the balance thereof being amylopectin. Amylose fractions are comprised almost exclusively of amylose while certain high amylose hybrid corn starches have an amylose content of about 40-70%.
The starch amylose content affects the film-forming, water-dispersibility and water-resistant properties of starch. Low-amylose starches are more easily convertible into aqueous pastes than high-amylose starches. Low-amylose starches are unacceptable as stable or permanent coatings because of their high water-sensitivity (e.g., readily swell and disperse into water). High-amylose starches are difficult to disperse and maintain as a uniform dispersion in aqueous systems (e.g., generally require 230.degree. F+ temperatures under superatmospheric pressure). Upon cooling (e.g., 200.degree. F. or less), the high-amylose starches readily retrograde into non-adhesive and water-insoluble starch particles. Unlike low-amylose starches, articles made or coated with high-amylose starches possess relatively good water-insensitivity and structural properties.
Considerable research has been expended towards the development of new techniques which would enable the art to use high-amylose, starch-based products as a synthetic polymer replacement. Representative thereof are U.S. Pat. Nos. 2,608,723 by Wolff et al.; 2,902,336 by Hiemstra et al.; 2,729,565 by O'Brian et al.; 2,973,243 by Kudera and 3,030,667 by Kunz; these patents basically disclose methods for preparing high-amylose shaped objects or coatings via organic solvent casting techniques. Shaped extrudates prepared by combining water, an organic plasticizer and high-amylose starches by extruding at elevated temperatures are reported in Canadian Patent No. 829,207. Notwithstanding these research efforts, high-amylose starches are still generally regarded as commercially unfit for coating and shaped object applications. Such processes generally involve physical manipulation of the high-amylose starch without altering or modifying the inherent compositional defects of the starch molecule. Moreover, the manipulative steps as well as their incompatibility with conventional coating and shaped article technology severely limit their adaption to commercial manufacturing processes.
At one time allyl starches appeared potentially useful as starch-based coatings (e.g., see J. P. Radley, Starch and Its Derivatives, 4th Ed., 1968). Unfortunately, the allyl starch coating systems have been plagued with difficulties such as non-homogenity, brittleness, inflexibility, poor water-resistance and limited solubility in organo solvent systems (e.g., see Polymerization Studies with Allyl Starch, Journal of Applied Polymer Science, Vol. 7, pages 1403-1410, 1963 by Wilham et al.). Hydrophobic, photopolymerizable N-methylol-polyol polymers have also been disclosed by Rosenkranz et al. in U.S. Pat. No. 3,936,428.